The present invention relates to the transmission of data over switched-network telephone channels.
The communication of data over telephone transmission facilities is conventionally carried out in either of two ways. One involves transmission of the data over a so-called private line in which the connection between transmitting and receiving equipment is always in place. The other involves transmission of the data over a switched-network, or "dial-up," channel.
The considerations relevant to the transmission of data over these two kinds of facilities are the same in many ways. For example, both typically require the use of a data set, or modem, the principal function of which is to modulate user-provided data into the passband of the telephone channel at the transmitting end and to demodulate the received data signals at the receiving end and recover the transmitted data. There are, however, some considerations unique to each. The present invention, in particular, is directed to certain problems which can arise in switched-network data transmission applications.
One of these problems arises as follows: Normally at the end of a call, the data processing equipment at one end of the channel, e.g., the equipment which initiated the call, transmits an end-of-call message to the equipment at the other end once the transfer of data has been completed. This serves as notice to the latter equipment that its associated telephone set should be disconnected from the telephone line. On occasion, however, the far-end equipment (e.g., central office) may never receive the end-of-call message. This can happen if, for example, the near-end telephone set is accidentally disconnected while communication of data is continuing. In that case, the telephone company's central office serving the near-end telephone transmits a disconnect signal to the central office serving the far-end telephone. Most central offices are designed to recognize the disconnect signal and will automatically disconnect the far-end telephone. However, some central offices do not recognize the disconnect signal. This leaves the far-end telephone indefinitely "busy", thereby precluding receipt of any further calls from other equipment unless the far-end telephone is hung up manually.
Another problem which may occur in switched-network data transmission arises as follows: In order for data to be efficiently exchanged over the switched-network at high speed, any echo suppressors in the transmission channel must be disabled. In Bell System facilities, for example, the echo suppressors are disabled in response to a 2025 Hz disabling tone transmitted by the called data set during call set-up. Thereafter the echo suppressors remain disabled as long as energy is maintained on the line. If energy is absent from the line for more than about 100 ms, the echo suppressors will re-enable. In order to ensure that this does not happen, one or both data sets transmits a holding tone whenever it detects a no-energy condition on the line. A problem arises, however, in circuits with long propagation times, e.g., satellite circuits. Suppose that a user transfers a calling station from the data mode to the talk mode by pushing the "talk" button on the telephone set. This action disconnects the signal processing circuitry of the data set from the telephone line and connects the telephone set thereto. With the calling data set disconnected from the line, only the called set can transmit the above-described holding tone. Assume further that the user causes energy to appear on the line by talking into the receiver. Upon detecting this energy, the called data set turns off its holding tone. This causes there to be an energy gap at the calling data set, and thus at its echo suppressor, for a time period which is a lesser of (a) duration of the voice energy and (b) the round trip propagation time of the channel. If that gap is greater than 100 ms, the echo suppressor at the calling set re-enables. Since the typical round trip delay on a satellite circuit is 600 ms, this problem is not uncommon on such circuits. Returning to the data mode will not re-disable the echo suppressors if the calling set was the one that was switched to the talk mode, because only the called data set transmits the disabling tone.